Conventionally, ultrasound diagnosis apparatuses have widely been used in today's medicine, because of being more compact than other medical image diagnosis apparatuses such as X-ray Computed Tomography (CT) apparatuses and Magnetic Resonance Imaging (MRI) apparatuses and because of being capable of displaying, in a real-time manner, an image of the target of a medical examination such as the heart or the liver through a simple operation of applying an ultrasound probe to the body surface of an examined subject. However, because an ultrasound image has a smaller field-of-vision area than an X-ray CT image or an MRI image, it is difficult to understand the three-dimensional position and orientation of the observed site, and also, the quality of the image varies depending on the examined subject (hereinafter, “subject”) and/or the person administering the medical examination.
To cope with this situation, in recent years, such an ultrasound diagnosis apparatus has been put into practical use that has a function of displaying an ultrasound image and another medical image (e.g., an X-ray CT image, an MRI image, or the like) taken on substantially the same cross-sectional plane as that of the ultrasound image, at the same time in a real-time manner. The ultrasound diagnosis apparatus generates, on a basis of position information of an ultrasound probe, the two-dimensional X-ray CT image or MRI image taken on substantially the same cross-sectional plane as the cross-sectional plane on which an ultrasound scan was performed, from X-ray CT volume data or MRI volume data, by performing a synchronized position alignment process on the positions of the images.
By using this function, for example, an operator is able to view, at the same time, the ultrasound image and the X-ray CT image taken on substantially the same cross-sectional plane as that of the ultrasound image. As a result, the operator is able to view the ultrasound image in a more comprehensible manner, in spite of ultrasound images generally having small field-of-vision areas, which make it difficult to understand the three-dimensional position of the observed site. For example, by viewing an X-ray CT image or an MRI image as a reference image at the same time with the ultrasound image, the operator is able to view a surrounding site in a wider area, the surrounding site not being depicted in the ultrasound image. As a result, the operator is able to recognize the three-dimensional position and orientation of the ultrasound image more easily. Furthermore, from the reference image, the operator is able to obtain morphological information of some parts that cannot be clearly recognized in the ultrasound image.
However, it is impossible to use the function described above, if the X-ray CT volume data or the MRI volume data of the subject who is subject to the ultrasound diagnosis process is not available.